Gravity, in the broad sense is defined as a vector force between the earth and a mass which is attracted to the earth. Being a vector force, gravity has three vector force components defined along mutually perpendicular directions. My earlier U.S. Pat. No. 4,290,307, issued Sept. 22, 1981, (hereinafter my "307" Patent) is directed to an apparatus which measures one of the component vectors, i.e., the vertical component. Another gravity instrument developed for measurement of the horizontal vector component of gravity is disclosed in my U.S. Pat. No. 4,271,702, issued June 9, 1981. The present invention takes the form of a gravity sensor which, though considerably different from the apparatus disclosed in U.S. Pat. No. 4,290,307, measures the vertical component of gravity and provides a more sensitive and efficient system yielding more efficient results as compared to the inventions disclosed in my previous patents.
Gravity is considered to be a three-dimensional vector force. Utilizing an X, Y, and Z-coordinate system, gravity can readily be defined as an attractive force between an object and the earth with the force having components in all three dimensions. If the coordinate system is conveniently defined at the center of the earth and the Y-axis is defined along the line between the earth and the object of interest, then the Y-component of gravity will be quite large compared to the X and Z-components. If they are conveniently defined as the north-south and east-west components (bringing into play the relatively well known surface coordinates on the earth), then those components are materially smaller. It will be appreciated that measurement of the Y or large component of gravity is extremely significant in certain scientific phenomena.
One phenomenon where gravity measurements of the earth are extremely helpful is in prospecting for minerals. The earth is not a homogeneous body. As a result, it is known that pattern variations in the measurement of the vertical component of gravity over a given geological region may very well show a set of variations which are coherently related to the geology of the region. As an example, large masses of iron ore create regional discontinuities in the measurements which, on proper interpretation, yield valuable information for determining the extent of the mass of iron ore in the earth.
While regional variations in gravity occur, variations also occur at a given locale over long or short periods of time as a result of a variety of reasons including, as an example, movement of extraterrestrial bodies. Accordingly, a set of base measurements over a period of time are usually deemed necessary to have a fixed base measurement whereby measurements taken in a large locale (for instance, in prospecting for various mineral deposits) are made so that all measurements can be referenced (by subtraction of time variations) to a common base station measurement to obtain time invariant measurements. To the extent that measurements at a given spot vary over a time interval, such variations are mathematically removed for the purpose of achieving a base station measurement taken in the locale. The present invention is a gravity meter which responds to variations in the vector component of gravity acting between the gravity meter and the earth and which converts such variations into a physical movement which can be measured and recorded on a time base chart.
The present invention has as one of its advantages a gravity measuring system using a hydraulically damped mass which mass moves in response to gravity variations. Such damping fairly well eliminates instrument system induced variations as might occur with an undamped structure. The mass moves responsive to variations over a period of time with sufficient damping so that overshoot, oscillations or transducer errors are not induced. The apparatus achieves this by utilizing a gravity attracted mass in a liquid bath.
During operation of the gravity measuring apparatus set forth in my "307" patent, one of the problems that has been experienced is the correlation of ambient temperature changes to the gravity measurements being taken. It is important that gravity measurement signals being transmitted to a computer system for signal processing, take into account the temperature at which the measurement was taken. Otherwise, gravity measurements uncompensated from the standpoint of temperature fluctuations, will exhibit errors. In the case of the apparatus of my "307" patent, changes in ambient temperature were rapidly conducted to the various components of the gravity measuring apparatus thus providing a requirement for temperature compensation. For example, during field operations a series of gravity measurements might begin early in the morning when the ambient temperature is quite cool. During the middle part of the day ambient temperature will typically increase, the ambient temperature being affected by the presence of wind, cloud cover and other natural phenomena. At each gravity measurement station the temperature at the time of measurement activity must be automatically or manually correlated with the gravity measurement thereby providing the computer system with appropriate temperature and gravity signals to insure that the graphical output of the computer plotter is accurately representative of subsurface anomalies. Though temperature related signals remain an important component for proper signal processing, the present invention is so compensated for temperature that temperature change within the measurement sensitive portion of the apparatus is quite slow and therefore gravity measurements between successive measurement stations will typically experience little or no temperature correction.
In a hydraulically damped gravity measurement system such as that shown in my "307" patent, movement of an internal float relative to detected changes in gravity are quite miniscule and therefore extremely sensitive position measuring apparatus are desired to achieve optimum results. In my earlier patent, miniscule gravity responsive movement of the float or tank 27 is multiplied by scale factors of from approximately 100 to 1000. This mechanical multiplier system provided sufficient measurement detection movement that a gravity change became more readily apparent. In accordance with the present invention, less complicated and more reliable position measurement equipment is utilized in accordance with the principles hereof to accurately detect and measure miniscule differences in the float position. In one form of the invention, gravity responsive positioning of a target is measured by an eddy current proximity sensor developing a position output signal that is digitized and displayed by a digital reader. A temperature responsive signal reflecting the temperature of the internal components of the system are also transmitted to and displayed by the digital reader. Field personnel are therefore enabled to simultaneously visualize internal temperature of the apparatus and gravity value. Also, if desired, these digital signals may be recorded on a computer disk for later processing to provide graphical representation of detected gravity signals.
In another form, position measuring apparatus may take the form of a laser activated system which is enabled to easily and quickly provide target position measurement signals to a degree of accuracy radically exceeding that of mechanical measurement apparatus. Other types of target positioning measurement systems, such as radar activated detectors may be deployed within the spirit and scope of the present invention.
From the foregoing, it will be understood how the apparatus is able to respond to variations in the vertical component of gravity which are converted into excursions of significant amplitude. They are converted and placed in a form enabling recording on a time based chart. Since the apparatus is temperature compensated and efficiently thermally insulated and may be set up at a measurement station and operated in a very short period of time, many successive stations may be measured without temperature compensation.